1. Field of the Invention
The present invention relates to a thermal head for thermal recording or thermal transfer recording and to a method of manufacturing the thermal head. More particularly, the present invention relates to a thermal head which is preferably utilized in a printer of a ticket vending machine, a printer for prepaid cards, a bar code printer, a printer for video signals, a transfer color printer, or a label printer.
2. Description of the Related Art
There are two typical types of thermal head utilized for thermal recording or thermal transfer recording. One is a plane type thermal head, and the other is an end face type thermal head.
The plane type thermal head is generally constituted by mounting an alumina substrate on which both an array of heating resistor elements and an IC chip for driving these heating resistor elements are formed, and a printed circuit board for electrically connecting the drive IC with an external circuit, on a plane surface of a heat sink formed by an aluminum plate.
The drive IC chip and bonding wires for connecting the IC with the heating resistor elements may be covered by protection resin. Over the resin, a cover fixed to the heat sink for securing the protection of the electrical connection between the IC and the printed circuit board is provided.
However, such the conventional plane type thermal head is difficult to be used for printing against a hard sheet such as a card (ex. cardboard, plastic card, metal card, etc.) because the card will be abutted to the cover which is projected over the IC and thus feeding of the card will be blocked. Also, such the thermal head has a problem that it is difficult to make it small in size. The latter is because that even if it is used for printing against ordinal flexible paper, the cover should be located apart from the heating element array so that a platen roller for pressing the paper to the heating element array does not touch the cover. Therefore, the substrate of the thermal head will become large in size. If the size of the head substrate is large, only a few number of the substrates can be processed in one sputtering process, carried out in a chamber, for forming a thin-film layer on the head substrate. As a result, the manufacturing cost of each thermal head will increase.
In each of publications of Japanese Patent Application Laid Open Nos. 57(1982)-83476, 59(1984)-146871, and 59(1984)-159365, a thermal head having a convex heating element array on a substrate to improve heat conductivity to a heat sensitive paper is described. However, since the projecting height of such the convex heating element arrays from the respective substrates are in the order of several tens .mu.m or less, there also exists a problem that feeding of the paper or hard sheet is blocked by a component such as a cover, mounted on the substrate.
In a publication of Japanese Utility Model Application Laid Open No. 58(1983)-60458, a thermal head having a convex heating element array the projecting height of which is higher than that of an drive IC is described. However, as the projecting height of this convex heating element array is about 0.5 mm or less, the above-mentioned problem of feeding is also occurred when a cover is attached over the drive IC. Furthermore, since both of side surfaces of this convex heating element array are formed to rise nearly perpendicularly from the substrate surface, conductive layer formed on these side surfaces by evaporation or etching process will become thin in thickness causing the conductive patterns to break off or to increase in resistance.
The end face type thermal head is one provided with a heating element array mounted on the end face of its heat sink. There are two types of end face type thermal heads, namely an L-shaped end face type thermal head and a curved end face type thermal head.
The L-shaped thermal head is generally constituted by mounting, on the end surface of a heat sink, an alumina substrate on which an array of heating resistor elements is formed, mounting a printed circuit board on the side plane surface of the heat sink, and electrically connecting between the circuit pattern in the alumina substrate and the circuit pattern in the printed circuit board by a TAB tape (film carrier) on which an IC chip for driving the heating resistor elements is mounted. The printed circuit board electrically connects the drive IC with an external circuit.
In such the L-shaped thermal head, a cover fixed to the heat sink should also be provided to cover over a junction between the circuit pattern of the alumina substrate and the film carrier on which the drive IC chip is mounted, for protecting the electrical connection. This cover causes, as similar to that in the plane type thermal head, feeding of a hard sheet to be printed such as a card to be blocked.
The curved end face type thermal head is substantially constituted by mounting, on the end of a heat sink made of aluminum for example, an alumina substrate provided with a curved end surface on which an array of heating resistor elements is formed, and on the side plane surface of which an IC chip for driving the heating resistor elements is mounted. The heating elements and the drive IC are electrically connected by bonding wires. A printed circuit board for electrically connecting the drive IC with an external circuit is mounted on the side surface of the heat sink, in the same side as that of the IC chip. Such the thermal head is known by publications of Japanese Patent Application Laid Open Nos. 55(1980)-103981, and 62(1987)-9965.
According to such the curved end face type thermal head, feeding of paper or hard sheet will be smooth without being blocked by any component mounted on the substrate. However, since it is necessary to form thin film layers on both the curved end surface and side surfaces, the manufacturing of the head is very complicated and thus takes a great deal of cost. Furthermore, according to such the curved end face type thermal head, since heat from the heating elements will be transmitted along the longitudinal direction of the head and thus the heat conduction pass in the aluminum substrate will be narrow in sectional area and long in distance, effective radiation of heat cannot be expected causing the thermal head to be easily excessive heat accumulation.